This invention relates to structures for increasing the breakdown voltage of PN junctions.
FIG. 1 illustrates a prior art diode 8 including a semi-insulating polycrystalline silicon (SIPOS) film 10 for enhancing the diode breakdown voltage. Diode 8 includes a P+region 12 serving as the anode and an N substrate 14 serving as the cathode. An N+region 16 facilitates electrical connection between substrate 14 and contact metallization 18, while P+region 12 electrically contacts metallization 20. (Although illustrated as two separate structures, metallization 18 is typically a single structure joined outside the cross section of FIG. 1. Similarly, SIPOS film 10 is a single structure and N+region 16 is a single region.) Of importance, SIPOS film 10 electrically contacts metallization 18 and 20 and extends over the PN junction of diode 8 between substrate 14 and P+region 12.
As is known in the art, a SIPOS film comprises oxygen doped polycrystalline silicon and exhibits high ohmic resistance. See, for example, the article entitled "Highly Reliable High-Voltage Transistors by Use of the SIPOS Process," by Matsushita et al., published in IEEE Transactions on Electric Devices in August 1976, incorporated herein by reference.
As is known in the art, when a diode such as diode 8 is strongly reverse-biased, film 10 serves to increase the breakdown voltage of diode 8. The reason for this is that current through resistive film 10 generates an electric field which alters the shape of the depletion region 24 between P+region 12 and N substrate 14. Specifically, this electric field increases the radius of curvature of depletion region 24, thereby enhancing the breakdown voltage of diode 8. Although SIPOS film 10 performs this function adequately, unfortunately, special equipment is required to form SIPOS films. This special equipment is typically expensive.